This invention is generally in the field of manufacturing multi-layer structures, such as semiconductor devices and integrated circuits, and relates to an optical measuring method and system for overlay measurements used to control the layer alignment in photolithography processing.
Integrated circuits are multi-layer structures produced by applying a sequence of deposition-lithography-etching steps to a semiconductor wafer. In such structures, layers have to be precisely aligned with each other. The layer alignment is typically controlled by the so-called xe2x80x9coverlay measurementxe2x80x9d, which is usually accomplished using a box-in-box (BIB) technique consisting of the following. A rectangular frame-like structure (target) is formed in a test site of each layer, and two locally adjacent layers (typically spaced by a dielectric layer) are considered as being correctly aligned if a specific alignment between the targets in the test sites is provided. Overlay defining the alignment is measured by comparing shifts between the targets along two mutually perpendicular axes, i.e., determining whether the targets are precisely concentric, the smaller target being inside the larger one (in projection along a vertical axis).
U.S. Pat. No. 5,438,413 discloses the use of a xe2x80x9cstand-alonexe2x80x9d apparatus for measuring overlay registration. This technique is employed for example in the xe2x80x9cstand-alonexe2x80x9d machine model KLA-5200 commercially available from KLA-Tencor. Such a stand-alone apparatus utilizes an interferometric microscope with a large numerical aperture operable to acquire a series of interference images at different vertical planes relative to the surface of a sample under investigation. Measured data in these planes is used for calculating the magnitude and phase of mutual coherence between the object and reference waves for each pixel in the image, thereby forming synthetic images. The brightness of this image is proportional to either the complex magnitude or the phase of the mutual coherence. The differences between synthetic images relating to the target position are then used as a means for detecting the overlay misalignment. The technique is characterized by a small depth of focus. Therefore, in most practical cases, two separate in-focus images of the two layers, respectively, are required. This unavoidably requires relative displacement between a focusing lens and a sample along the optical axis of the lens. This procedure is very sensitive to vibrations, since a very small lateral shift between the two images caused by the vibrations would be recorded as a misalignment between the layers. Therefore, such a system requires the use of a comprehensive vibration isolation platform, and accordingly is practically impossible to use as an integrated measurement system, which has to be of a small size and has to be stable to a high vibration level during measurement caused by a processing tools arrangement.
U.S. Pat. No. 6,166,801 assigned to the assignee of the present invention, discloses another technique of the kind specified, according to which an integrated measurement system is used operating with a relatively small numerical aperture, and consequently a larger depth of focus, as compared to the above-described technique of U.S. Pat. No. 5,438,413. Here, the focal plane of the objective is brought to a position at one of the two locally adjacent layers under measurement or at a location between the two measured layers. This technique allows for capturing a common image of both targets in two locally adjacent layers, respectively, wherein each frame is located at a specific position with respect to the focal plane of the objective. The technique, while being sufficiently good for the structures where the layers are spaced from each other a distance up to 1 xcexcm, appears to be less effective when this distance exceeds 1 xcexcm, since in this case aberrations of an optical system (e.g., coma, chromatic and astigmatic aberrations) become more significant and affect the measurement accuracy.
There is accordingly a need in the art to facilitate controlling layer alignment by providing a novel method and system for optical overlay measurement.
The present invention utilizes the acquisition of two images of two target structures in two layers, of a sample in a single measurement, at the same location of the focal plane of an objective relative to the sample. This is implemented by designing and operating the measurement system such that the two targets are imaged on either a common or different imaging planes, the imaging plane being a conjugate plane with respect to each of the two layers, respectively, providing the best image quality for each of the respective targets. To achieve this, light components coming from the two targets and collected by the common objective lens are directed towards the imaging plane(s) along two different optical paths, respectively.
There is thus provided according to one aspect of the present invention, a method for imaging two target structures in two parallel layers, respectively, of a sample, the method enabling determination of a registration between the two target structures along two mutually perpendicular axes of the layer, the method comprising:
(a) illuminating the sample with incident radiation to produce a radiation response of the sample to said incident radiation;
(b) collecting the radiation response by an objective lens arrangement;
(c) splitting the collected radiation response into two spatially separated radiation components and directing the split radiation components towards at least one imaging plane along different optical channels characterized by optical paths of different lengths, respectively;
(d) detecting said two split radiation components in said at least one imaging plane, thereby concurrently acquiring two image parts, each containing images of the two target structures, and enabling determination of the relative distance between the two target structures.
The term xe2x80x9cradiation responsexe2x80x9d used herein signifies at least one of the following: reflection of the incident radiation, scattering of the incident radiation, radiation excited by the incident radiation (e.g., luminescence or fluorescence).
The different optical paths may be predetermined by carrying out a calibration procedure and/or based on knowledge about the sample""s structure (recipe), and/or may be adjustable in accordance with the quality of the image being acquired. Preferably, the best image quality is considered as that characterized by the optimal image contrast and/or optimal image resolution. The splitting of the collected radiation response may utilize a regular beam splitter (typically, a partially transparent mirror), a polarized-based beam splitter, or a notch- or wavelength-selective splitter.
According to one embodiment, the two images are captured by a common pixel-array detector (e.g., CCD camera), i.e., in the common imaging plane. In this case, the images can be acquired in different regions of the sensing surface of the detector, or within the same sensing region of the detector. According to another embodiment, two pixel-array detectors are used for acquiring the two images of two target structures, respectively, i.e., two different imaging planes.
According to another aspect of the present invention, there is provided an optical measurement system for imaging two target structures in two different layers, respectively, of a sample, the system comprising:
an illumination device operable to produce incident radiation for illuminating the sample and thereby producing radiation response of the sample to the incident radiation;
an objective lens arrangement accommodated so as to collect the radiation response;
a detection system comprising at least one pixel-array detector defining at least one imaging plane;
a radiation directing assembly accommodated in the path of the collected radiation response and operable to split the collected radiation response into two spatially separated radiation components and direct the separated radiation components to said at least one imaging plane along two different optical channels characterized by optical paths of different lengths, respectively, thereby enabling to concurrently acquire on said at least one imaging plane two image parts, each comprising images of the two target structures.
More specifically, the present invention is used for overlay measurements for controlling a photolithography process applied to a semiconductor wafer, and the operational principles of the technique of the present invention allow for designing an integrated measurement system. Such a system is intended for measurements in samples (e.g., wafers) progressing on a production line, and is therefore designed to be installed within a processing machine outside the processing area.